1. Field of the Invention
The present invention relates to an electron emission material and an electron emission panel having the same. More particularly, the present invention relates to an electron emission material that can be used both as a cold cathode and a thermionic cathode, and an electron emission panel having the same.
2. Description of the Related Art
Generally, electron emission devices use a thermionic cathode or a cold cathode as an electron emission source. The Edison effect is a term used to describe the emission of electrons from electron emission devices using the thermionic cathode. The Edison effect describes a phenomenon whereby electrons are emitted from the surface of a metal or a semiconductor heated to a high temperature. That is, electrons are emitted from the surface of a solid when the electrons acquire sufficient energy to be emitted from an atom. The Edison effect is also called the Richardson effect or thermal electron emission phenomenon, and has been applied to a vacuum tube or a discharge tube, also generally known as a thermionic tube, as applied to vacuum tubes for various communication systems, Coolidge tubes (an X-ray tube), etc.
Electrons emitted from the surface of a metal or semiconductor through the Edison effect may be referred to as thermal electrons and a material that emits the electrons may be referred to as a thermionic cathode. A current generated by the migration of the thermal electrons from a thermionic cathode toward an anode, which may be disposed to face the thermionic cathode and may have a high positive (+) voltage, may be referred to as a thermal electron current or an electron emission current.
Generally, the electron emission current from a thermionic cathode increases as the temperature increases. However, the electron emission current may vary according to the kind of thermionic cathode or the surface of the thermionic cathode. For example, alkali earth metal oxides may emit electrons at a lower temperature than pure metals. The cathode of a small vacuum tube, which may be used at a relatively low voltage, may be formed of, e.g., tungsten or tungsten mixed with thorium, and there are many examples that use a metal oxide of these metals. Also, the Edison effect depends on the work function of a material, which describes the energy required to free an electron from a material. Tungsten emits electrons at a temperature of approximately 2427° C. (a work function of approximately 4.5 eV), and thorium-tungsten emits electrons at a temperature of approximately 1627° C. (a work function of approximately 2.6 eV).
Electron emission devices that use a cold cathode include Field Emission Device (FED) type devices, Surface Conduction Emitter (SCE) type devices, Metal Insulator Metal (MIM) type devices, Metal Insulator Semiconductor (MIS) type devices, Ballistic electron Surface Emitting (BSE) type devices, etc.
FEDs use the principle that, when a material having a low work function or a high β function is used as an electron emission material, the material readily emits electrons in a vacuum due to an electric field formed between two or more electrodes. FEDs have been developed that employ a tapered tip structure formed of, e.g., Mo, Si, etc. as a main component, a carbon group material such as graphite, diamond like carbon (DLC), etc., or a nano structure such as nanotubes, nanowires, etc.
However, FEDs that use carbon nanotubes may be unsatisfactory for some applications, in that the lifetime of the electron emission material may be short and the electron emission may be small. Commercialization of FEDs has been hampered because they tend to exhibit low current densities. When a display device is manufactured from an electron emission device that emits few electrons, brightness and light emission efficiency of the display device may be low. Therefore, there is a need to develop an electron emission material having good electron emission efficiency, and an electron emission panel having the same.